Electron discharge device having cathode protective means within the envelope



g- 1966 R. K. GESSFORD, SR 3,

ELECTRON DISCHARGE DEVICE HAVING CATHODE PROTECTIVE MEANS WITHIN THEENVELOPE Filed March 28, 1962 INVENTOR Ross K. G255 ford, 5

ATTORNEY United States Patent 3,267,321 ELECTRON DISCHARGE DEVICE HAVINGCATH- ODE PROTECTIVE MEANS WITHIN THE EN- VELOPE Ross Gessford, Six,Seneca Falls, N.Y., assignor to Sylvania Electric Products Inc., acorporation of Delaare Filed Mar. 28, 1962, Ser. No. 183,246 6 Claims.(Cl. 315--3) axially aligned, closely spaced electrodes normallydesignated as G1, G2, G3, G4, and G5 formed to provide the modulation,focussing, and acceleration of the electron beam.

The spacing between adjacent ones of these electrodes can be consideredinherently to define a given sparking potential in the evacuated tubewhich is higher than the normal operating potential gradient betweenthese electrodes. However, it is not uncommon that transient highvoltage surges originating with the high potential electrodes beimparted to the lower potential adjacent electrodes through disruptivedischarges or arc-overs. This discharge often originates between the G3and G2 electrodes because of the high potential difference normallyexisting therebetween. Once the discharge occurs, it will then progressto the G1 and then to the cathode, which is insulatingly mounted insidethe G1. This disruptive discharge, when applied to the cathode, is oftensufficient to destroy at least part of the emissive coating and therebyimmediately cause or accelerate failure oi the tube.

Heretoifiore, efforts have been made to protect the tube filament duringarcing. The protecting devices used for this purpose consisted of sparkgaps or shorting straps interconnecting a grounded heater filament leadand the cathode or control grid. Although some protection was affordedthe filament, these arrangements have proven unsatisfactory since theydo not provide any protection for the cathode.

There has also been suggested the provision of a high voltage surgesuppression circuit generally employing a condenser grounded at one sideand connected at the other side to a tube electrode which may besubjected to high voltage surges. Although some suppression occurs inthese devices, the tube operation is disrupted [for an undesirableperiod of time rfrom its normal tfunctioning. lFurther, the suppressioncircuit does not prevent internal arcing between many electrodes anddoes not protect the tube cathode.

It is, theretore, an object of this invention to protect the emissivecoating of a cathode [from high voltage surge damage.

It is another object of this invention to afford voltage surgeprotection for the tube and the operating circuit while minimizing thetime period of disruption [from normal tube operation.

In accordance with the \foregoing objects there is provided by thisinvention a transient high voltage surge bypass device incorporatedwithin a cathode ray tube. This device is fabricated, according to oneaspect 01f the invention, from an electrically conductive material andhas one terminal portion connected to ground potential, while the otherterminal portion is connected to the G2 electrode and provides asparking potential lower than the sparking potential existing betweenthe G1 and G2 electrodes.

Accordingly, when a disruptive discharge occurs between the G2 and G3electrodes, instead of progressing to the G1 and the cathode closelyassociated therewith, it will be conducted to ground by way out theby-pass device. By preventing these disruptive discharges from reachingthe cathode, .a substantial increase in the life of the emissive coatingis achieved, thereby increasing the life of the tube. Also, because thedischarge ctr-om the G2 to the by-pass device is in the form of animmediate arc, the time period of disruption from normal tube operation,caused by th abnormally high G2 potential, is minimized.

For purposes of this invention, the spark-ing potential between a givenpair of electrodes in a gas is considered to depend upon the length ofthe spark gap and the pressure of the gas in such a way that it isdirectly proportional to the mass 0t gas between the two electrodes.Also, the work function of a given metal is referred to as the work perunit charge, expressed as a voltage, which is required to free anelectron from the influence ot the charges in the metal and thus toprovide escapement from it.

Since the spaced electrodes described herein form an electron gun havingprescribed parameters and since the low gas or air content and pressurewithin the evacuated tube are common to all electrodes, the sparkingpotential or work function potential can appropriately be referred to asbeing dependentupon the spacing between electrodes (for comparisonpurposes.

For a better understanding of the present invention, together with otherand turther objects, advantages, and capabilities thereof, reference ismade to the iollowing disclosure and appended claims in connection withthe accompanying drawings in which:

FIG. 1 is a partial diagrammatic view of a cathode ray tube employingone aspect of the invention;

FIG. 2 illustrates two spaced electrodes embodying an alternate form ofthe invention; and

FIGS. 3 and 4 show additional embodiments of the invention.

In FIG. 1 there is shown generally a cathode ray tube '10 comprising anevacuated, hermetically sealed envelope 12 with an electron gun 14mounted therein. Gun 14 employs a cathode assembly 16 having a sleeve'18 with emissive coating 20 on the end thereozE and a heater 22partially mounted within the sleeve.

Positioned forward of emissive material 20 are a plurality of axiallyaligned, spaced electrodes designated as G1, G2, G3, G4, and G5. The G1or control grid has the cathode assembly 16 insulatingly mounted withinit by means of ceramic wafer 24. For a grid driven tube, G1 may have avideo drive range of about 45 volts relative to the cathode. The G2serves as the screen grid and G4 is utilized as a focussing ring for theelectron beam. These electrodes may be operated at about 450 and 300volts respectively while the G3 and G5 electrodes,

-which serve as the accelerating electrodes and cooperate with G4 forfocussing purposes, are maintained at a relatively high potential whichmay reach 20 kv. or higher. In order to maintain the proper alignmentand spacing necessary for operation, these electrodes are provided withpins 26 for attachment to a plurality of insulating support rods 28,only one of which is shown in the draw-. mg.

While the spacing between any two adjacent electrodes ingun 14determines the sparking potential therebetween as pointed out above, ofprimary interest here is the sparking potential across gap 30 which isdefined as the spacing between the first electrode G1 and the secondelectrode G2. Gap 30 is of interest since it is the closest electricalisolating medium to the cathode to G1 sparking potential gap. It will beappreciated that under normal conditions,

if an arc occurs between the high voltage electrode G3 and low voltageelectrode G2, there exists momentarily a potential difference betweenelectrodes G1 and G2 which may be in the order of.20,000 volts. Sincethis voltage far exceeds the normal G1 to G2 sparking potential, an arcwill occur therebetween, thus imparting the high voltage surge to G1.Almost instantly then the high volt-age Surge is transferred to cathode18 because of its close proximity to G1. Of course, the surge voltagedifference between the cathode and G1 vastly exceeds the normal sparkingpotential therebetween. Since electron emissive material 20 is on thesurface of cathode 18 closest to G1, this material is inherently in thedischarge zone and thereby becomes deleteriously disrupted andcontaminated by the discharge.

In order to protect cathode 18, an transient high voltage surge by-passdevice 32 is contained within envelope 12 and is mounted, as by welding,to interconnect the second electrode G2 and a ground potential lead 34.The by-pass device 32 is fabricated from an electrically conductivematerial preferably of ribbon stock or wire, and it is formed to providea sparking potential across space" 36 which is lower than the sparkingpotential across gap 30. Accordingly, any are to G2 will pass over space36 to ground rather than to cathode 18.

The material from which by-pass device 32 is fabricated may be the sameas that used for electrodes G1 and G2, for instance a nickel alloy, orit may be chosen from one of the materials known as cold emitters. Theseinclude materials as for example titanium, magnesium, and barium-nickelalloy, which have -in common a relatively low work function. When one ofthe above-mentioned materials is used, it should be chosen so that itswork function potential is lower than the sparking potential acrossspace 30.

Referring now to FIG. 2, there is shown an alternate ems bodiment of thebypass device. This transient high voltage surge by-pass device 38comprises an encapsulating, hermetically sealed envelope 40 ofinsulating material surrounding a gaseous atmosphere, and a pair ofelectrically conductive members 42 and 44 fixedly mounted in the wall ofencapsulating envelope 40. Members 42 and 44 are afiixed, as by welding,to electrode G2 and ground potential lead 34 respectively and projectinwardly of encapsulating envelope 40 to terminate in spaced plates 46,48. The nature of the gas content, together with the pressure andspacing, provide a predetermined sparking potential across gap 36 whichis lower than the sparking potential across space 30. Such a device hasthe advantage of isolating the discharge and ionization from theevacuated tube atmosphere.

In another embodiment shown in FIG. 3, members 42 and 44 of the by-passdevice 50 are fixedly mounted in the upstanding Walls 52 of asubstantially U-shaped insulating block 54. The sparking potential ofthis device is determined by space 36 and it is less than the sparkingpotential across space 30. Device 50 may be separately constructed in anaccurate manner prior to assembly with the gun electrode G2.

FIG. 4 illustrates another embodiment wherein a bypass device 56comprises a cylindrical block 58 of insulating material having alongitudinal bore 60 and opposed electrically conductive plates 62 and64, having spherical protuberances 66 and 68 bonded to block 58. Plates62 and 64 are affixed, as by welding, to conductive members 42 and 44respectively. The protuberances 66 and 68 are formed to project intobore 60 to provide a space 36 therebetween which determines a sparkingpotential lower than the sparking potential across space 30. Here again,this device may be separately constructed and it has the additionalcapability of confining the discharge and resulting ionizationtherewithin.

, It will be apparent that a by-pass device electrode, preferablyconductive member 44, in any of the embodiments, may also be formed froma material having a low work function potential as above described.

While the preferred determinant for the appropriate 1 sparking potentialhas been illustrated in conjunction with space or gap 30 between the G2and G3 electrodes, it will be appreciated that other interelectrode gapsmay serve as an appropriate determinant in the illustrated electron gunor in other types of guns so long as the cathode is properly protected.

The invention described herein provides a simple economical, rugged,device that will effectively protect the emissive coating on a cathodefrom transient high voltage surges, thereby enhancing the lifecharacteristics of the tube in which it is employed.

While there has been shown and dscribed what are at present consideredthe preferred embodiments of the present invention, it will be obviousto those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:

1. A cathode ray tube comprising an evacuated, hermetically sealedenvelope, an electron gun positioned within said envelope, said electrongun having a cathode and at least first and second sequentially spacedlow voltage electrodes, said second electrode being subject to transienthigh voltage surges, the spacing between said first and secondelectrodes defining a given sparking potential, at least one leadterminating interiorly of said evacuated envelope and projectingexteriorly for connection to ground potential, and a voltage surgeby-pass device mounted within said envelope interconnecting said secondelectrode and said ground potential lead, said bypass device beingformed to provide a predetermined sparking potential lower than saidgiven sparking potential.

2. A cathode ray tube comprising an evacuated, hermetically sealedenvelope, an electron gun positioned within said envelope, said electrongun having a cathode and at least first and second sequentially spacedlow 7 voltage electrodes, said second electrode being subject totransient high voltage surges, the spacing between said first and secondelectrodes defining a given sparking potential, at least one leadterminating interiorly of said evacuated envelope and projectingexteriorly for connection to ground potential, and a voltage surgeby-pass device mounted within said evacuated envelope comprising anencapsulating, hermetically sealed envelope of insulating materialsurrounding a gaseous atmosphere, a pair of electrically conductivemembers fixedly mounted in the wall of said encapsulating envelope andinterconnecting said second electrodes and said ground potential lead,said pair of members being formed to project inwardly of saidencapsulating envelope and terminating in a spaced relationship toprovide a predetermined sparking potential therebetween lower than saidgiven sparking potential.

3. -A cathode ray tube comprising an evacuated, hermetically sealedenvelope, an electron gun positioned within said envelope, said electrongun having a cathode and at least first and second sequentially spacedlow voltage electrodes, said second electrode being subject to transienthigh voltage surges, the spacing between said first and secondelectrodes defining a given sparking potential, at least one leadterminating interiorly of said evacuated envelope and projectingexteriorly for connection to ground potential, and a voltage surgeby-pass device mounted within said evacuated envelope comprising asubstantially U-shaped block of insulating material having a baseportion and a pair of upstanding walls projecting therefrom, a pair ofelectrically conductive members fixedly mounted in said upstanding wallsand interconnecting said second electrode and said ground potentiallead, said pair of members being formed to project inwardly toward thecenter of said block and terminating in a spaced relationship to providea predetermined sparking potential therebetween lower than said givensparking potential.

4. A cathode ray tube comprising an evacuated, hermetically sealedenvelope, an electron gun positioned within said envelope, said electrongun having a cathode and at least first and second sequentially spacedlow voltage electrodes, said second electrode being subject to transienthigh voltage surges, the spacing between said first and secondelectrodes defining a given sparking potential, at least one leadterminating exteriorly of said evacuated envelope and projectingexteriorly for connection to ground potential, and a voltage surgeby-pass device employing a cold emitter mounted Within said evacuatedenvelope and interconnecting said ground potential lead and said secondelectrode, said cold emitter having a work'function potential lower thansaid given sparking potential.

5. A cathode ray tube comprisinga-n evacuated, hermetically sealedenvelope, an electron gun positioned within said envelope, said electrongun having a cathode and at least first and second sequentially spacedlow voltage electrodes, said second electrode being subject to transienthighvoltage surges, the spacing between said first and second electrodesdefining a given sparking potential, at least one lead terminatinginteriorly of said evacuated envelope and projecting exteriorly forconnection to ground potential, and a voltage surge by-pass devicemounted within said envelope interconnecting said ground potential leadand said second electrode comprising a cylindrical block of insulatingmaterial having a longitudinal bore therethrough, opposed electricallyconductive plates having spherical protuberances thereon bonded to saidblock, said proturberances projecting into said bore to provide a spacetherebetwee-n, said space defining a sparking potential lower than saidgiven sparking potential.

6. A cathode ray tube comprising an evacuated, hermetically sealedenvelope, an electron gun positioned Within said envelope, said electrongun having a cathode and a plurality of intermediate spaced low voltageelectrodes axially aligned with and sequentially arrayed from saidcathode, at least one of said intermediate electrodes being subject totransient high voltage surges, the spacing between said one of saidintermediate electrodes and an adjacent electrode defining a givensparking potential, at least one lead terminating in-teriorly of saidevacuated envelope and projecting exteriorly for connection to groundpotential, and a voltage surge by-pass device mounted within saidenvelope interconnecting said one of said intermediate electrodes andsaid ground potential lead, said by-pass device being formed to providea predetermined sparking potential lower than said given sparkingpotential.

Y References Cited by the Examiner UNITED STATES PATENTS HERMAN KARLSAALBACH, Primary Examiner.

ARTHUR GAUSS, Examiner.

S. OHATMO'N, JR., Assistant Examiner.

1. A CATHODE RAY TUBE COMPRISING AN EVACUATED, HERMETICALLY SEALEDENVELOPE, AN ELECTRON GUN POSITIONED WITHIN SAID ENVELOPE, SAID ELECTRONGUN HAVING A CATHODE AND AT LEAST FIRST AND SECOND SEQUENTIALLY SPACEDLOW VOLTAGE ELECTRODES, SAID SECOND ELECTRODE BEING SUBJECT TO TRANSIENTHIGH VOLTAGE SURGES, THE SPACING BETWEEN SAID FIRST AND SECONDELECTRODES DEFINING A GIVEN SPARKING POTENTIAL, AT LEAST ONE LEADTERMINATING INTERIORLY OF SAID EVACUATED ENVELOPE AND PROJECTINGEXTERIORLY FOR CONNECTION TO GROUND POTENTIAL, AND A VOLTAGE SURGEBY-PASS DEVICE MOUNTED WITHIN SAID ENVELOPE INTERCONNECTING SAID SECONDELECTRODE AND SAID GROUND POTENTIAL LEAD, SAID BYPASS DEVICE BEINGFORMED TO PROVIDE A PREDETERMINED SPARKING POTENTIAL LOWER THAN SAIDGIVEN SPARKLING POTENTIAL.